Noise suppression system for telephone lines



31.11 339 11936, E. AUBORT NOISE SUPPRESSION SYSTEM FOR TELEPHONE LINES Filed Sept. 8, 1933 Patented June 30, 1936 UNITED STATES PATENT OFFICE NOISE SUPPRESSION SYSTEM FOR TELEPHONE LINES Edouard Aubort, Baden, Switzerland, assignor to Aktiengesellschaft Brown,

Boveri & Cie.,

In Great Britain 9 Claims.

This invention relates to a noise suppression system for telephone lines, and particularly to methods of and circuit arrangements for eliminating interference from adjacent power lines and other alternating and direct current systems.

When the telephone wire or wires have portions adjacent and approximately parallel to a high voltage power transmission system, relatively heavy currents may be induced in the telephone wire or wires. The eiiect of such induced currents is to build up a potential difference between the ends of the telephone line and, for convenience of description, these potentials will be termed longitudinal potentials. To elimimate the induced longitudinal potentials, or the effect of such potentials upon the transmission of the telephone signals, it has been proposed to shield the wires by a cable casing or by grounded protective conductors adjacent the telephone wires. Another arrangement suitable for a two wire system is to connect the wires, at their terminals and at one or more intermediate points,

| by coils having grounded center taps. By thus reducing the impedance between the individual wires and ground, the longitudinal voltages are short-circuited to ground.

Disturbing voltages may be established in the local telephone circuits which are coupled to the telephone line, due both to direct interference from the power line and to the longitudinal voltages induced in the telephone wires. Such disturbing voltages will be designated noise voltages, since they are present at the local stations and cause disturbing noises or 5 interference at the telephone receiver. Attempts have been made to eliminate noise voltages by coupling the local network to the telephone wires through choke coils, thus introducing a compensating coupling between the telephone wires and the power lines which create the disturbance.

It has been proposed to employ the longitudinal voltages to compensate the noise voltage by connecting the ends of telephone lines by a coil, the center of the coil being connected to 45 ground through a resistance which is coupled to the local or station network. Although such compensations are satisfactory so long as the insulation symmetry of the telephone lines from ground remains unchanged, the longitudinal potentials vary with the insulation conditions and therefore the compensating voltage developed across the resistance varies. As the noise voltage does not vary directly with the longitudinal voltages, frequent adjustments of the system are necessary, especially with overhead wires where November 12, 1932 the insulation resistance is greatly affected by weather conditions.

Objects of the invention are to provide methods of and circuit arrangements which avoid the objections noted above, and in which the noise compensation is substantially independent of the insulation resistance of the line conductors. More particularly, objects are to provide methods of and circuit arrangements in which the longitudinal potentials aredischarged to ground through 10 a center tap on a transformer winding connected across the line conductors, and a parasitic short-circuit current to ground establishes a noise compensating voltage in an impedance network connected between the sections of a split 15 secondary transformer winding in the subscribers circuit.

These and other objects and advantages of the invention will be apparent from the following specification, when taken with the accompanying drawing in which:

Figs. 1, 2 and 3 are fragmentary circuit diagrams of telephone lines embodying the invention.

In the drawing, the reference characters A, B identify the conductors of a two wire telephone line which is located adjacent a power line M that causes disturbance and noise in the reception of telephone signals. If the line is relatively long, the longitudinal voltages induced in the line may be substantially reduced by the capacity of the line to ground or, in accordance with known practice, one or more discharge coils D, preferably of the transformer type, are connected across the line at appropriate intervals, the center taps of the coils being connected directly to ground, as shown, or through condensers of large capacity.

In accordance with this invention, the primary winding P of a transformer is connected between the conductors A, B, and the sections S of the split secondary winding are included in the local circuit of the subscriber, the equipment at the subscribers station being indicated at L. The center of the primary P is connected to ground through a lead G and the primary winding of a noise compensation transformer N, and the secondary winding is connected between the sections S of the first transformer. The transformer N has a low leakage reactance and the windings are therefore closely coupled. A variable resistance R is shunted across the secondary of transformer N to provide an adjustment of the amplitude of the available compensating voltage that is introduced into the local circuit.

It will be apparent that as the insulation conditions Vary, the changes in longitudinal potentials will be reflected in a change in the noise voltage components transferred to the iocal circuit by'transformer P, S and in similar changes in the noise compensation voltage introduced 7 into the local circuit bythe transformer N.

As shown in Fig. 2, other forms of coupling may be substituted for the transformer N, The windings of the main transformer may be the same as in the Fig. 1 circuit, but a fixed resistance R is serially connected between the sections S of the secondary, the center of resistance R. being connected to the center tap of the primary P. Theground lead G is connected to a slider that is adjustable along resistance R and may be positioned at either side of the fixed center tap. The ends of resistance R maybe connected' directly to the sections S of thesecondary or, as illustrated, a condenser C may be included in each lead. "These condensers are not subjected to high potentials and may be of the standard telephone design which employs paper as the dielectric. V

The noise compensation network may be so designed'that there will beno flow of direct ourrent to ground from either the wires of the telephone line or the local circuit. As shown in Fig. 3, thecircuit may be similar to that of Fig. 2,

j except that the center of the primary winding P is opened for. the insertion of two iarge condensers C and the lead to resistance R is connected to the junction of the cc-ndensers C. The

condensers are preferably electrolytic and of v such high capacity as to have a negligible im 'pedance' for telephone signals and for currents V of'the disturbing longitudinal potentials.

The diiferences in the longitudinal voltages which result in' a disturbing or noise voltage are mainly due to variations in the insulation conditions of the telephone lines as the insulation symmetry of the two lines may be upset by a variety of causes, such as'weather effects, trees and other obiects'adiacent the lines, faults between the lines, etc. The magnitudes of the longitudinal voltages are reduced substantially to associated ground connections. therefore arise from slight differences in the short-circuit currents to ground and, in accord- 'ance with the invention, these minute short-cirzero by means of the discharge or repeater coils with grounded midpoints which are disposed at the ends of the aifected sections and, preferably, also at. points between the ends, as'is shownin Fig. 1. Although this mid-point grounding of the lines through low impedance connections to earth provides a compensation for insulation variation which would appear to reduce the noise voltages to zero, it is a fact that there will generally be some slight differences in the impedances of the different telephone wires and their Noise voltages cuit currents are employed to introducela compensating voltage into the local circuit. jDue to the fact that the longitudinal voltages at the mid-point of the primary winding P are reduced 'to zero, or'substantiallyto zero, the difference between the longitudinal voltages in the two wires is negligible and is therefore independent offany lack of insulation symmetry in the two wires.

7 connection from the center of said primary wind- 7 7 ing to ground, circuit elements connected between the sections of said split secondary, and means 10 coupling said grounding connection to said circuit elements to establish noise-compensating potentials in said local circuit by the flow of parasitic currents through said grounding connection.

2. In a telephone system including a telephone line comprising a pair of line conductors, and a local circuit, the combination with a trans- :former having a primary winding connected be I tween said pair of line conductors and a split secondary winding included in the local circuit,

of a connection from the center of said primary winding to ground, and a transformer having the primary thereof included in said grounding connection and the secondary connected between the sections of said split secondary.

3.. The invention as claimed in claim 2, in combination with an adjustable resistance connected between the sections of said split secondary and in shunt with the secondary of said second trans former. 1

4. In a telephone system including a telephone line comprising apair of line conductors, and a local circuit, the combination with a transformer having a primary winding connected between said pair of line conductors and a split secondary winding included in the local circuit, of a resistance connected between the sections of said split'secondary, a connection from the center of said resistance to the center of said' primary winding, and a connection from said resistance 40 to ground. V V e 5. The invention'as set forth in claim 4, wherein the connection from said resistance to ground 7 includes a slider adjustable along said resistance.

Theinvention as set forth in claim 4, wherer in the connections between said resistance and one of the said windings include condensers for blocking the flow of direct current.

The invention as set forth in claim 4, wherein said primary winding includes two equal sections serially connected by a pair of condensers of negligible impedance, and the connection from said primary winding. to said resistance is made to the junction of said condensers.

8. The invention as set forth in claim 4, wherein the connections between said resistance and each of the said windings include condensers for blocking the flow of direct current.

9. The method of reducing noise voltages in V telephone systems of the type including a pair 'of line'conductors coupled'to a local circuit 1 through a transformer, and a ground connection to the'mid-point of the primary winding of the transformer; which method comprisesreducin the impedance of the ground connection substantially to zero, and employing the parasitic currents flowing in'said ground connection to impress upon the local circuit compensating voltages neutralizing the noise voltages existing therein. 7 i

EDOI IARD AUBORT. 

